Systems and methods for defrost of heat pump systems

A multi-circuit heating, ventilation, and air conditioning (HVAC) system for use with a first refrigerant and a second refrigerant, the multi-circuit HVAC system comprising:

• a first refrigerant circuit for the first refrigerant comprising a first compressor, a first outdoor heat exchanger, a first expansion device, and an indoor heat exchanger;
• a second refrigerant circuit for the second refrigerant, the second refrigerant circuit being fluidically isolated from the first refrigerant circuit and comprising a second compressor, a second outdoor heat exchanger, a second expansion device, and the indoor heat exchanger;
• and
• a first defrost circuit connected to and branching from the first refrigerant circuit in two positions, the first defrost circuit comprising a first three-way valve, a first defrost line, a first defrost passage in the first outdoor heat exchanger, and a first defrost return line;
• wherein when the HVAC system is in a defrost mode:
• the first three-way valve is operable to divert some or all of the first refrigerant from the first refrigerant circuit and through the first defrost circuit to defrost the first outdoor heat exchanger;
• and
• the first defrost return line returns the diverted first refrigerant to the first refrigerant circuit upstream of the first expansion device and downstream of the indoor heat exchanger.

A method of defrosting a heating, ventilation, and air conditioning (HVAC) system operable to use a refrigerant to heat or cool an indoor space, the method comprising:

• compressing the refrigerant with a compressor;
• flowing the refrigerant in a refrigerant circuit between an indoor heat exchanger an expansion device and an outdoor heat exchanger, wherein the refrigerant flows through the expansion device in a first direction in a cooling mode and in a second direction, opposite the first direction, in a heating mode;
• and
• wherein in a defrost mode:
• flowing the refrigerant in the second direction in the refrigerant circuit;
• actuating the first three-way valve to divert some or all of the refrigerant from the refrigerant circuit and through a defrost line, a defrost passage in the outdoor heat exchanger, and a defrost return line;
• defrosting the outdoor heat exchanger with the diverted refrigerant in the defrost passage;
• and
• flowing the diverted refrigerant in the defrost return line to return the diverted refrigerant to the refrigerant circuit upstream of the expansion device and downstream of the indoor heat exchange with respect to the second direction.